1. Field
The present invention relates to a method and apparatus for generating a lightwave comprising several frequencies from a fundamental RF frequency. More specifically, the present invention relates to a method for generating a lightwave with at least two frequencies having a frequency separation equal to the atomic hyperfine frequency of an atom.
2. Description of Related Art
Precise timekeeping is important in communications and navigation satellite systems. As a result atomic clocks can play an important role in such systems because of their long-term accuracy in monitoring time. Similarly, timekeeping is critical for the Global Positioning System (GPS), where atomic clocks are deployed on satellites, because small time errors can lead to significant positioning errors. Because these clocks are often deployed on satellite systems, it is also very desirable to reduce the power consumption of the clock.
Atomic clocks are based on transitions between atomic energy levels. In one particular type of clock, cesium (Cs) atoms are exposed to a lightwave having two different frequencies that stimulate two different transitions. These transitions are separated in frequency by the atomic hyperfine frequency of Cs (approximately 9.192 GHz).
The National Institute of Standards and Technology (NIST) teaches one approach for generating two frequencies separated by the atomic hyperfine frequency separation of Cs. In the NIST approach a laser is amplitude modulated with a modulation frequency equal to half of the Cs atomic frequency, about 4.596 GHz. In order to generate the modulation frequency, NIST takes a fundamental frequency that can be easily generated and accurately measured, typically around 100–200 MHz, and uses frequency multipliers, amplifiers, mixers, filters, and dividers to generate the modulation frequency. The modulation frequency is then used to amplitude modulate a laser source.
Amplitude modulating the laser at 4.596 GHz creates a lightwave in the frequency domain, as shown in FIG. 1. The lightwave contains a center frequency f0 which is typically about 352.112 THz (corresponds to a 852 nm wavelength) and a sideband pair having an upper sideband f12 and a lower sideband f23. It is well known that the upper and lower sidebands produced by amplitude modulating a laser are separated from the center frequency f0 by a frequency equal to the modulation frequency. This results in the upper sideband f12 and lower sideband f23 having the necessary hyperfine frequency separation f13 of 9.192 GHz needed to stimulate Cs atoms.
The NIST approach provides an all-optical atomic clock that does not use the injection of separate microwave signals at the Cs atomic frequency into a microwave excitation cavity, as required by other prior art atomic clocks. A drawback with the NIST approach is that the RF-circuitry needed to generate the modulation frequency requires too much power (5–20W) and space to be used for lower power applications. That is, as noted above, several frequency multipliers, etc. are needed to upconvert the fundamental frequency to the modulation frequency of the amplitude modulated signal.
As a result, there is a need for a method and apparatus which can generate a lightwave or lightwaves having two frequencies separated by the atomic hyperfine frequency of an atom species, without using power consuming RF-circuitry.